<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        input{
            color:#999;
        }
    </style>
</head>
<body>
    <input type="text" value="手机">
    <script>
       var text = document.querySelector('input');
       //获得焦点  判断表单里面是否为默认文字 如果是默认文字 就清空表单内容
       text.onfocus = function(){
        if(this.value === '手机'){
            this.value = '';
        }
        //获得焦点 需要把文本框里的文字颜色变黑
        this.style.color = '#333';
       }
       text.onblur = function(){
        if(this.value === ''){
            this.value='手机';
            this.style.color='#999';
        }
       } 
    </script>
</body>
</html>